1. Field of the Invention
The present invention relates to an optical converter for generating, in response to an input optical pump beam from a laser, multi-wavelength optical output signals with variable wavelength distribution.
2. Description of the Related Art
Stimulated Raman scattering (SRS) was discovered in 1962, and has been extensively studied in atomic and molecular gasses, numerous liquids, and solids. The generation of tunable coherent radiation by means of stimulated Raman techniques is widely employed as a method for creating intense radiation over a wide range of wavelengths. A basic treatise on stimulated Raman scattering is found in "Tunable Lasers", by J. C. White, Springer Series Topics in Applied Physics, vol. 59, Springer, Berlin, Heidelberg, 1987, pp. 115-207.
It is desirable in certain applications to generate an optical signal which contains a plurality of component waves of different wavelengths, and to be able to vary the distribution of the wavelengths in the signal. Vibrational SRS in a single Raman cell in response to an input optical pump beam from a laser generally generates a single Stokes wave with a relatively large frequency shift. Rotational Raman scattering (SRRS) produces a smaller frequency shift than vibrational SRS, enabling the generation of two, or possibly three Stokes shifted waves. However, increasing the intensity of the input pump beam in an attempt to generate more Stokes shifted waves causes optical breakdown of the Raman medium in the cell at a lower intensity than that at which more waves would be generated.
Where the Raman medium is a gas such as hydrogen, methane, or deuterium, the pressure can be varied to selectively promote the generation of rotationally or vibrationally shifted waves. However, a substantial length of time is required to produce pressure variation, and at higher pressure levels which support vibrational SRS, various other phenomenon, such as stimulated Brillouin scattering (SBS), compete with SRS for pump energy. Thus, the generation of an optical signal with a variable distribution of more than two or three component wavelengths has not been achieved utilizing SRS.